Optical phase-change disc

ABSTRACT

An optical phase-change disc comprises a substrate having thereon a spiral groove or concentric grooves for guiding a focused light beam, and a layer structure including a recording layer and protective layers sandwiching therebetween the recording layer. The groove has wobble for recording ATIP (absolute time information) or ADIP (address information). The following relationship between the groove width GW, beam diameter R 0  and wobble amplitude a w  : 
     
         0.25≦GW/R.sub.0 ≦0.45 
    
     or 
     
         0.65≦GW/R.sub.0 ; 
    
     and 
     
         0.03≦a.sub.w /GW≦0.08 
    
     hold for preventing distortion of the groove caused by repeated overwriting operation to improve reliability of the optical disc.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The invention relates to a high density rewritable phase-change changeoptical storage media, and more particularly, to a phase-change opticalstorage media which exhibits a reduced degradation during repeatedoverwriting.

(b) Description of the Related Art

Recently, an increase in the amount of information data demands ahigh-density storage media which permits a recording/playback of a vastamount of information rapidly. It is expected that optical storage mediawould meet the need of such applications.

An optical disc includes a write-once type which allows a recordingoperation only once and a rewritable type which allows an overwriting asmany times as desired. A rewritable optical disc includes amagneto-optical disc which utilizes the magneto-optical effect and aphase-change disc which utilizes a change in the reflectivity associatedwith a reversible phase-transformation between crystallized andamorphous states. A phase-change disc does not require an externalmagnetic field and enables a recording/erasure by merely modulating thepower of a laser irradiation, thus presenting an advantage that arecording/playback unit can be constructed in a compact size. It alsoaffords an advantageous possibility that a higher density can beachieved by using a irradiation source of shorter wavelengths, withoutmodifying materials used in a recording layer or the like in aconventional disc recorded and erased with a currently dominantwavelength on the order of 800 nm.

A material for a recording layer of phase-change type often comprises athin film of chalcogen alloy such as GeSbTe, InSbTe, GeSnTe, AgInSbTeetc., for example. In a rewritable recording disc of phase change typewhich is currently implemented for practical use, a unrecorded (orerased) state is represented by a crystallized state, whereas recordedstate is represented by an amorphous state. The amorphous bit is formedby heating the recording layer to a temperature higher than the meltingpoint, followed by quenching. To prevent an evaporation and/ordeformation from occurring as a result of such heat treatment of therecording layer, the recording layer is usually sandwiched byheat-resistant and chemically stable dielectric protective layers whichare disposed on the opposite sides thereof.

During a recording process, the protective layers promote a thermaldiffusion from the recording layer to achieve a suprer-cooled condition,thus contributing to the formation of an amorphous bit. A metallicreflective layer is generally provided on the sandwich structure toprovide a quadri-layer structure, which further promotes the thermaldiffusion to insure the amorphous mark formation. Erasure (orrecrystallization) takes place by heating the recording layer to atemperature above the crystallization temperature, but below the meltingpoint. In this instance, the dielectric protective layers act as heataccumulating layers.

For a so-called one-beam overwritable phase-change disc, both theerasure and re-recording process can be simultaneously achieved by theintensity modulation of a single focused light beam. (See J. Appl.Phys., 26(1987) Suppl. 26-4, pp.61-66.) With the one-beam overwritablephase-change disc, the layer construction of the recording disc and thecircuit arrangement of the drive can be simplified, thus drawingattention for its use as an inexpensive high density and high capacityrecording system.

The recording process for the phase-change disc involves an extremethermal stress cycle that forcibly melts the recording layer and thenquenches it below the melting point within several tens of nanoseconds.For this reason, even if the recording layer is sandwiched by thedielectric protective layers, a repeated overwriting operation as manyas several thousands or several tens of thousands times builds up amicroscopic deformation or segregation in the recording layer,eventually leading to an increase of optically recognizable noise andthe formation of local defects of micron order size. (see J. Appl.Phys., 78(1995), pp.6980-6988.) While a substantial improvement isachieved through a sophistication in respects of the recording layermaterial, the material for the protective layers or layer structure,there is an essential upper limit on the number of overwritingoperations, which is by one order of magnitude or more below the numberof overwriting operations available with a normal magnetic recordingdisc, or magneto-optical recording disc.

The degradation which results form the repeated overwriting operationsdepends on the configuration of a groove. To give an example, arewritable compact disc (CD-Rewritable or CD-RW) is recently proposed("CD-ROM Professional" in the United States, Sep. 1996, pp.29-44 orAssembly of Manuscripts for Phase-Change Optical Recording Symposium,1995, pp.41-45.).

For a CD, a train of pits formed in a substrate at a pitch of 1.6±0.1 μmis scanned from the back side of the substrate by a focused light beamhaving a wavelength of 780±30 nm to retrieve information. It isprescribed that the reflectivity in a non-pit area be equal to orgreater than 70%.

For CD-RW, although the compatibility with CD inclusive of as high areflectivity as 70% or more is difficult to achieve, the compatibilitywith CD can be secured in respect of the recorded signal and groovesignal as long as the requirement for the reflectivity is allowed to beabove 15% and below 25% for a non-recorded area and below 10% for arecorded area. The compatibility can be secured within the reach of thecurrent CD drive technology if an amplification system which compensatesfor a reduced reflectivity is added in a playback system.

In a CD-RW, the groove is used as a recording track, and a record ismade within the groove. It is proposed that a wobble containing addressinformation can be used in the groove (JP-A-1993-210,849). FIGS. 1A and1B illustrate a schematic view of such a disc. Wobbled grooves 11 arespaced apart in the surface of substrate and are separated from eachother by inter-groove (land) spaces 2. It is to be noted that theamplitude of the wobble is shown exaggerated. The wobble is formed by afrequency modulation using a carrier frequency of 22.05 kHz. The wobbleamplitude is very small in comparison to the pitch of the groove 11,which is a distance measured between imaginary centerlines of grooves 11located on the opposite side of the inter-groove space 12 and isnormally on the order of 1.6 μm, and is on the order of 30 nm.

A frequency modulation of the wobble in accordance with absolute timeinformation or address information is referred to as ATIP (Absolute TimeIn Pre-groove) or ADIP (Address In Pre-Groove), and is already utilizedin a recordable compact disc (CD-Recordable or CD-R) and mini-disc. (See"CD Family" by Heitaro Nakajima, Takao Ihashi and Hiroshi Ogawa,Published from OHM-sha in 1996, Chapter 4, and Proceedings of the IEEE,Vol. 82(1994). Page 1490.)

It is found by an investigation by the present inventors that repeatedoverwriting operations produce a new degradation phenomenon that thedeteriorated wobble signal leaks into the recorded signal. This furtherreduces the repeatable times, by one order of magnitude or more, down tothe order of 1000. In a CD-RW, the wobble is used to provide an addressinformation which is required in detecting a unrecorded region intowhich information is to be recorded. This phenomenon, which limits thenumber of times for the overwriting operation, will present seriousproblem when the wobble is used in a disc having a high track density.

On the other hand, it will be seen that what is most frequently recordedin repeated manner in a usual recording disc will be a rewriting of filemanagement area which stores content information for user data, and itis unlikely that the user data itself is rewritten as many as 1000times. By way of example, considering a CD format, it is likely that TOC(Table Of Contents) region or PMA (Program Memory Area) region in arewritable CD disc is frequently rewritten. Such a file management areais a small limited region which is disposed along the innermost oroutermost periphery of the entire recordable region of the optical disc,and remains to be less than several percents of the entire recordableregion; a degradation attributable to the wobbled groove presents aproblem mainly in the TOC region as far as the CD format is concerned.However, this represents a very important region in which the content ofthe user data is recorded. Once an error occurs in this region, thereresults a failure to read data from the entire data area, and the disccan no longer be used, thereby limiting the life of the disc. To copewith this problem, a reserve track may be secured for use as asubstitute for the file management area so that the substitute track maybe used when an increased number of errors occur as a result ofoverwriting operations. However, still there is a limit on the number ofoverwriting operations that can be used, and in addition, a procedure ofthe file management is troublesome, presenting a difficulty in thedesign for drives and device drivers. In other words, the actualcircumstance is that the number of overwriting operations which can berepeated for the entire disc is limited due to the presence of a regionwhich is frequently rewritten and which occupies less than severalpercents.

SUMMARY OF THE INVENTION

According to a first aspect, the invention resides in an opticalrecording disc comprising: a substrate having a spiral groove orconcentric grooves which meanders in accordance with a modulation signaland has a depth equal to or greater than 25 nm and less than 200 nm andwhich are used to guide a focused light beam; and at least three layersincluding a lower protective layer having a thickness of 70 nm and lessthan 200 nm, a rewritable recording layer of phase-change type, and anupper protective layer having a thickness equal to or greater than 10 nmand less than 60 nm, consecutively formed on the substrate, wherein thewobble signal has a carrier level to noise ratio (C/N ratio) equal to orgreater than 25 dB, and wherein wobble amplitude a_(w), a diameter R₀ ofthe focused light beam across the groove and a groove width GW satisfythe following relationships:

    0.25≦GW/R.sub.0 ≦0.45 or 0.65≦GW/R.sub.0( 1)

    0.03≦a.sub.w /W≦0.08                         (2)

According to a second aspect, the invention resides in an opticalrecording disc comprising: a substrate having a spiral groove orconcentric grooves for guiding a focused light beam; and a rewritablerecording layer of phase-change type disposed on the substrate, the dischaving a data area and a file management area along the groove orgrooves, wherein the configuration of the groove is deformed inaccordance with an address signal and a synchronization signal in thedata area, and the configuration of the groove remains constant in thefile management area.

According to a third aspect of the invention, the invention resides inan optical recording disc comprising: a substrate having a spiral grooveor concentric grooves for guiding a focused light beam; a rewritablerecording layer of phase-change type disposed on the substrate, the dischaving recording tracks along the groove or grooves, wherein the datatracks includes user data area and additional data area disposedalternately along the recording tracks, wherein the configuration of thegroove is deformed in accordance with address data and synchronizationdata in the additional data area, and the configuration of the groove isconstant in the user data area.

In accordance with the invention, in an optical recording disc ofphase-change type having a wobble which generally causes problemdegradation due to repeated overwriting operation, the degradation issuppressed to improve the reliability and the durability of the opticaldisc.

The signal deformation of the groove geometry as used in this text isreferred to as deformation or modulation of the groove that is appliedto the wobble configuration or width or depth of the groove inaccordance with address data, synchronization data or other specificdata. The modulation is implemented by vibrating the exposure beam on aglass master in manufacturing process of a stamper in the directionnormal to the direction of the groove during the exposure of the glassmaster, and subsequent transcription of the vibration onto the substrateby injection molding. The typical deformation of the groove isimplemented by the wobble of the groove. In contrast, the constantgroove as used herein means that the groove is not applied with thewobble configuration or modulation and extends in a constantconfiguration.

The present invention achieves advantages of suppression of degradationin a phase-change disc caused by repeated overwriting operations toimprove reliability and durability of the phase-change disc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a wobble and FIG. 1B is a cross-sectional viewtaken along line A--A' in FIG. 1A;

FIG. 2 is a cross-sectional view of an exemplary layer structure of anoptical disc according to the present invention;

FIG. 3 is a plan view of the wobble of FIG. 1A and a focused opticalbeam irradiating thereto;

FIG. 4 is a schematic waveform of a servo error signal;

FIGS. 5A and 5B are examples of timing charts for modulated recordingpower;

FIG. 6 is a schematic cross-sectional view showing areas in CD andrecordable CD;

FIG. 7 is schematic plan view of an optical recording disc according toan embodiment of the invention;

FIG. 8 is a schematic plan view of a conventional magneto-optical disc(MO) showing arrangement of sectors;

FIG. 9 is a plan view of a CD-RW for showing data tracks according to anembodiment of the invention;

FIG. 10 is a block diagram of a disc exposure unit for a prototype glassdisc;

FIGS. 11A, 11B and 11C are schematic side view, plan view and intensitydistribution, respectively, of a laser beam;

FIG. 12 is a graph illustrating the relationship between 3T jitter andthe number of overwriting operations for wobble amplitudes;

FIG. 13 is a graph showing a relationship between 3T jitter and thenumber of overwriting operations for widths of wobbled grooves;

FIG. 14 is a graph showing a relationship between 3T jitter and thenumber of overwriting operations for widths of grooves having no wobble;

FIG. 15 is a cross section showing the definition of the groove width;

FIG. 16 is a graph showing a relationship between the C/N ratio andrecording power in sixth embodiment;

FIG. 17 is a graph illustrating a relationship between C/N ratio andrecording power in seventh comparative example;

FIG. 18 is a graph showing energy distribution of a focused laser beam;and

FIG. 19 is a graph showing fluctuation caused by groove wobble inretrieved envelope signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be more specifically described based onpreferred embodiments thereof with reference to the accompanyingdrawings.

The invention uses a technique to record a frequency-modulated signal inaccordance with a rotational synchronization pattern or addressinformation in the form of a groove wobble, as disclosed, for example,in JP-A-2(1990)-87344. The groove wobble can be formed by oscillating anexposure beam for groove formation in the direction normal to the grooveduring fabrication of a prototype glass master in mastering process. Alarge number of replicas can be manufactured by transferring the shapeof the glass master onto resin substrates by using an injection moldingtechnique (see, for example,JP-A-1(1988)-103454,-2(1990)-87344,-2(1990)-198040 and 3(1991)88124, and-3(1991)-237657, JP-B-3(1991)-23859 and -3(1991)-3168).

ATIP or ADIP signal recorded or described by the wobble is used in thecontrol of the rotational speed in the unrecorded region and addressingin the data area. ("Compact Disc Dokuhon" by Heitaro Nakajima andHiroshi Ogawa, published from OHM-sha in 1988 and Patent Publicationsmentioned above). It is to be noted that the wobble sometimes comprisesa carrier frequency without utilizing the frequency modulation, and usedonly for establishing a rotational synchronization of the disc.

The present inventors have found that a degradation resulting from arepeated overwriting operations is promoted in a CD-RW (rewritablecompact disc) of phase-change type by the presence of the wobble, andnoted that the degradation causes a more serious problem in the futurewhen a higher track density is used. The inventors have also found thata promoted degradation can be suppressed under a specific condition.

The structure of the optical disc including a recording layer ofphase-change type and a recording method according to the principle ofthe invention will now be described. A substrate may comprise atransparent resin such as polycarbonate, acrylic resin or polyolefin, orglass. A recording layer of phase-change type has its both sides coatedwith protective layers. It is desirable that the disc has a layerstructure as shown in FIG. 2 including lower protective layer 14 of adielectric material, recording layer 15, upper protective layer 16 of adielectric material and reflective layer 17 consecutively formed on asubstrate 13. The top of the disc may be preferably coated with aprotective overcoat 18 comprising ultra-violet ray curable orthermosetting resin. The reflective layer 17 is provided in order totake advantage of an optical interference effect positively to therebyincrease the signal amplitude and to provide a function as a heatdissipating layer to thereby assist in achieving a super-cooledcondition required to form an amorphous mark. At this end, it isdesirable to choose a metal having high reflectivity and high thermalconductivity, such as Au, Ag and Al, for the reflective layer 7.However, a semiconductor such as Si, Ge or the like may be used in orderto make a design choice in some instance. From the standpoints ofeconomical considerations and corrosion resistance, it is desirable tochoose an Al alloy to which from 0.5 to 5 atomic % of Ta, Ti, Cr, Mo,Mg, Zr, V, Nb or the like is added. In particular, the addition of Taprovides a high corrosion resistance (JP-A-1(1989)-169751).

The protective layer 14 disposed on the surface of the substrate has athickness in a range from 10 to 500 nm. The choice of a material for theprotective layers 14 and 18 is determined in consideration of refractiveindex, thermal conductivity, chemical stability, mechanical strength,adherence to other layers and the like. Generally, the oxides sulfidesand nitrides of metals or semiconductors and fluorides of Ca, Mg, Li orthe like, which are highly transparent and has a high melting point, canbe used. It is unnecessary that these oxides, sulfides, nitrides andfluorides have a stoichiometric composition, but the composition may becontrolled or a mixture may be used in order to control the refractiveindex or the like. A dielectric mixture is preferable when the repeatedrecording response is considered. More specifically, a mixture of ZnS orrare earth sulfide and a refractory compound such as oxides, nitrides orcarbides is preferable. It is desirable from the standpoint of themechanical strength that the film density of such a protective layer beequal to or greater than 80% of the bulk (see "Thin Solid Films", Vol.278 (1996), pp. 74-81).

The dielectric layer having a thickness below 10 nm may be insufficientto prevent a deformation of the substrate or recording layer as aprotective layer. If the thickness is above 500 nm, internal stresseswithin the dielectric layer itself and the differential elastic responsewith respect to the substrate become remarkable, tending to producecracks. In particular, it is preferable to suppress a deformation of thesubstrate due to heat by the lower protective layer, and a thicknessequal to or greater than 70 nm is preferable for this purpose. Below athickness of 70 nm, a microscopic deformation of the substrate isaccumulated during the repeated overwriting operations, whereby areproduced light is scattered causing a considerable increase of noise.An upper limit on the thickness of the lower protective layer issubstantially on the order of 200 nm in consideration of the depositiontime. If the thickness of the lower protective layer is larger than 200nm, the configuration of the groove as viewed in the plane of therecording layer will be changed, which is undesirable. Specifically, thedepth of the groove may become shallower than intended on the surface ofthe substrate, and the groove width may also be narrower than intendedon the surface of the substrate, both of which are undesirable. Apreferred upper limit on the thickness of the lower protective layer is150 nm or less.

On the other hand, a thickness of at least 10 nm or more is required forthe upper protective layer 16 in order to suppress the deformation ofthe recording layer. If the thickness is greater than 60 nm, there is atendency that a microscopic plastic deformation is accumulated withinthe upper protective layer during the repeated overwriting operations,and this causes a reproduced light to be scattered, thereby increasingundesirable noise.

As mentioned previously, the recording layer in the disc of theinvention is of phase-change type, and has a thickness which ispreferably in a range from 10 nm to 100 nm. If the thickness of therecording layer is less than 10 nm, a sufficient contrast cannot beobtained, and there is also a tendency to retard the recrystallizationrate, presenting a difficulty in erasing a record in a short timeinterval. On the other hand, if the thickness exceeds 100 nm, an opticalcontrast is difficult to achieve, and a crack is likely to be produced,which is again undesirable. For practical purposes, a thickness equal toor greater than 10 nm and below 30 nm is used in order to assure a highcontrast which provides a compatibility with CD. Below 10 nm, thereflectivity is too low, whereas above 30 nm, a heat capacity increasesto degrade a recording sensitivity.

A recording layer may be an optical recording layer of phase-change typewhich is known in the art, and may comprises a compound such as GeSbTe,InSbTe, AgSbTe or AgInSbTe, for example, is selected as an overwritablematerial. In particular, a thin film comprising as a main constituentthe following alloy:

    {(Sb.sub.2 Te.sub.3).sub.1-x (GeTe).sub.x }.sub.1-y Sb.sub.y (0.2<x<0.9, 0≦y<0.1)

or

    M.sub.w (Sb.sub.z Te.sub.1-z).sub.1-w (0≦w<0.3, 0.5<z<0.9),

and

where M represents at least one selected from the group comprising In,Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr, Co, O, S, Se, V, Nb andTa is stable in either crystallized or amorphous state, and allows arapid phase transition between the both phases. Such a material is amost practical in view of the advantage that it is less susceptible tosegregation after repeated overwriting operations. The recording layeris generally obtained by sputtering an alloy target in an inert gas, inparticular, Ar gas.

The thicknesses for the recording layer and protective layers 15 areselected for a desirable absorption efficiency of laser irradiation andan increased amplitude of recorded signal, i.e., a better contrastbetween an recorded and an unrecorded states, in consideration of aninterference effect caused by a layer structure in addition to therestrictions imposed by the mechanical strength and the reliability.

As mentioned previously, the recording layer 15, the protective layers14 and 16 and the reflective layer 17 are formed by a sputteringtechnique.

To prevent an interlayer oxidation and contamination, it is desirablethat a thin film deposition process is conducted in an in-line equipmentincluding a vacuum chamber in which a target for the record film, atarget for the protective films, and if desired, a target for thereflective layer are disposed in common. This is advantageous from thestandpoint of productivity.

In general, in the phase-change disc, microscopic deformation isaccumulated in the protective layers or the surface of the substrate dueto repeated overwriting operations, thereby scatters the focused opticalbeam to increase noise in the reproduced light or to change thethickness of the recording layer and protective layers, which retardaccurate detection of the mark length.

The degree of degradation due to the repeated overwriting operationsdepends on the geometry of the groove. The present inventors found thatthe progress of the degradation due to the repeated overwritingoperations is low in a constant groove having a larger depth and asmaller width in case of recording in the groove, and that theconfiguration of the groove is determined thereby.

The reason therefor is considered to result from a confinement effect ofthe recording layer. That is, it is believed that the deeper andnarrower the groove, the more the melted region is limited in the grooveduring recording operation to suppress the width of the deformed regionin the bottom of the groove during the meltdown of the recording layer.

The distortion of the groove geometry due to the overwriting operationapplies to the groove wall. The groove wall suffers from thermal damagedue to the poor adherence to the thin film and stress concentration atthe corner during the repeated overwriting operation. Accordingly, evenif only a part of optical beam is irradiated onto the groove wall,degradation will be promoted. In particular, with a disc in which thegroove 11 is formed in a resin substrate or in a photosetting resin, adistortion in the groove geometry which results from the repeatedoverwriting operations occurs more or less because the softening pointof the resin is far below the temperature of the phase-change discduring the phase-change operation, the temperature being several hundreddegrees centigrade or higher.

The measurements of the distortion in the bottom and wall of the grooveby an atomic force microscope (AFM) exhibited that protrusions anddepressions on the order of 2 to 3 nm are formed after repeatedoverwriting operations, generating distortion on the groove wall. Thus,the overwriting durability of the groove limits the range of the groovewidth which is determined relative relationship between the groove widthGW and the diameter R_(o) of the focused optical beam as viewed in thedirection normal to the groove. As shown in FIG. 18, the energydistribution of the recording focused optical beam is determined by aGaussian distribution, and the degree of the degradation is determinedby the portion of the Gaussian distribution actually applied to thegroove wall. The diameter R_(o) of the focused beam in the directionnormal to the groove as used herein is referred to as the diameter atwhich the intensity of Gaussian beam is 1/e².

In the state wherein an optical energy which is well below the peak ofthe Gaussian beam irradiates the groove wall, the degradation of thegroove wall is negligible as a mater of course. The critical lightintensity irradiated to the groove wall is approximately 40% of thelight intensity at the center of the groove, and the groove wall shouldbe located outside the critical location. The critical light intensitycorresponds to 0.65=GW/R₀ in the Gaussian distribution.

To further examine the physical meaning of this fact, temperaturedistribution in the recording layer is investigated. The calculation forthe temperature distribution resulted in up to the temperature of 1000°C. at the beam center during irirradiation for forming an amorphousmark.

Since the thermal conductivity of the recording layer and protectivelayers is below that of Al by two or three orders, a first orderapproximation formula can be used wherein the temperature distributionneglecting the thermal conductivity follows the Gaussian distribution.By this approximation, below approximately 400° C. is obtained at theradius 0.65×R₀, which indicates that the temperature of the groove wallis well below the melting point (500°-700° C.) of the recording layer asused in the phase-change disc. If the groove width is narrowed from thisradius, the degradation due to the irirradiation to the groove wallincreases. However, it is found in our research that degradation, i.e.,increase of noise, is rather suppressed in the range GW/R₀ ≦0.45 due tothe confinement effect of the recording layer by the groove wall.

To summarize the above, the following relationship:

    GW/R.sub.0 ≦0.45, or 0.65≦GW/R.sub.0         (1)

is necessary in FIG. 18, in order to improve the overwriting durabilityof the groove recording. If the groove width is too large, it leads toretard the tracking servo operation and reduce the density of tracks.Accordingly, it is preferable to follow the relationship GW/R₀ <1.0.

Further, we found that if the groove width is too small in the case ofW/R_(O) ≦0.45, degradation occurs due to the presence of wobble, whichdetermines the lower limit of the groove width or GW/R₀.

While the mechanism that the degradation is promoted by the presence ofthe wobble is not clearly understood, it is considered that the presenceof the wobble tends to cause the focused light beam 19 which is used forthe recording operation, to irradiate partly a sidewall 20 of thegroove, as illustrated in FIG. 3. Specifically, the beam 19 to whichtracking servo feedback is applied does not accurately follow thewobble, rather passes straightforward along the centerline 21 of thegroove 11. Accordingly, the light beam 19 tends to irradiate the groovewall 20 even though slightly. Obviously, the wobble amplitude a_(w) isshown exaggerated in FIG. 3 but it is believed that the tendencyillustrated is correct.

In general, the wobble amplitude is on the order of 1 to 10 nm, andaccordingly, the distortion of the groove in the order of 2 to 3 nmconsiderably degrades the wobble signal quality. In this case, not onlyC/N (carrier to noise) ratio degrades, but also S/N (signal to noise)ratio for the signal recorded in the groove degrades. Observed signalwaveform indicated, as shown in FIG. 19, that envelope of the recordedsignal oscillates in accordance with the wobble of the groove, whichvibration becomes noticeable with a narrower groove and a larger wobbleamplitude. It was noticed that the degradation of the wobble resulted inthe irregularity of the cyclic change of the envelope, which in turnresulted in larger noise leaked in the recorded signal.

Ultimately, if the wobble is not provided to the groove, the degradationas described above will not arise. Further, if an amorphous mark doesnot deviate from the groove, the groove width should be narrow enough sothat GW/R₀ ≦0.45, or wide enough to neglect the degradation of thegroove wall so that 0.65<GW/R₀ in order to reduce the degradation due tothe repeated overwriting operations.

According to our study, it is not preferable that an excessive lightintensity is applied to the groove wall in FIG. 18, because degradationin wobble signal due to the groove wall distortion is more dominant thanthe confinement effect of the recording layer by the narrow groove inthe case of presence of wobble. It disposes a lower limit upon thegroove width which is 0.25≦GW/R₀. After all, in view of the repeatedoverwriting durability of the recording layer, the groove width GWshould satisfy the following relationship:

    0.25≦GW/R.sub.0 ≦0.45, or 0.65≦GW/R.sub.0.(1)

In FIG. 18, if GW/R₀ ≦0.25, degradation of the envelope following thewobble is dominant, and if 0.45≦GW/R₀ ≦, 0.65, degradation is moreprominent because of two factors including the degradation by groovedistortion and insufficient confinement effect by the groove wall. If0.6≦GW/R₀, repeated overwriting durability is recovered because of thenegligible groove distortion although confinement effect by the grooveis not provided.

The degradation caused by the wobble depends also on the wobbleamplitude a_(w). Specifically, the larger the ratio of wobble amplitudea_(w) to groove width GW, i.e., a_(w) /GW, or the larger density ofprotrusions and depressions the groove had, the more remarkable thenoise leaked into the record signal due to the distortion of the wobble.In this respect, the configuration a_(w) /GW≦0.08 is essential toprevent the degradation of envelope due to the wobble distortion. On theother hand, an extremely smaller wobble amplitude does not provide asufficient signal intensity for the wobble signal.

Moreover, the wobble amplitude has a lower limit because C/N of thewobble signal should be equal to or above 25 dB and an upper limitbecause the degradation caused by repeated overwriting operations, asfollows:

    0.03≦a.sub.w /GW≦0.08                        (2)

The value is determined experimentally. This relationship depends not onthe wavelength of optical beam or NA, but on beam diameter R₀ in thedirection normal to the direction of the groove and relativerelationship between wobble amplitude a_(w).

The amplitude of the groove wobble as used in the present invention isextremely difficult to measure directly with an electron microscope orscanning probe microscope. Accordingly, in accordance with theinvention, the wobble amplitude is defined by a measurement as describedbelow:

Specifically, in a given optical head, the relationship between adisplacement a_(w) from the mean center of the groove as viewed in FIG.3 and a wobble signal amplitude I_(w) is given in terms of off-trackamount of a tracking servo system and a servo error signal, as follows:

    I.sub.w =A·sin (2·π·a.sub.w /p)(10)

where p represents a predetermined track pitch or a distance from thecenter of a land located on one side of the groove to the center ofanother land located on the other side thereof. The wobble amplitude ofthe groove, i.e., the displacement from the mean center of the groovecan be determined from this equation.

FIG. 4 shows a servo error signal. The factor "A" appearing in theequation (10) represents half the peak-to-peak amplitude of servo errorsignal from a push-pull system, as measured without the use of thetracking servo or in an open loop, and is given as follows:

    (I.sub.1 -I.sub.2).sub.pp =2·A                    (11)

Iw can be determined as the amplitude of the tracking servo error signalwhich is obtained when a tracking servo operation is applied to thewobbled groove. In this manner, by determining the servo error signal,the values of p, A, I_(w) can be measured and substituted in to theequation (3) to determine a_(w). With the technique described above,a_(w) can be determined in principle independently from the response ofan optical head, the beam configuration and the groove geometry. It isto be understood that such technique of measurement itself is well knownin the art.

The above description is given in terms of CD-RW as an example, which ischosen because a clear definition of terms is available. However, itshould be noted that the invention is not limited to the current CD, butis also applicable to a recording a disc having a higher density whichis constructed similarly to the current CD where a rotationalsynchronization signal can be generated and an address signal may beproduced by utilizing the groove wobble.

It is to be noted that there is a preferred range for groove depth inaccordance with the invention in view of the repeated overwritingdurability. Generally, a phase-change disc exhibits a more excellentdurability against overwriting operations in an ingroove recording thanin an inter-groove (or on-land) recording. While the reason therefor isnot clearly understood, it is believed to be a result of an effectiveprotection of an edge area of a recording layer by the groove wall. Sucha protective effect (groove confinement effect) is not satisfactory fora groove depth below 25 nm. On the other hand, for a groove depth whichexceeds 200 nm, the adherence of a sputtered film to the groove wallbecomes difficult, causing a reduced film thickness on the wall or adegraded film of a reduced density to be formed, which is undesirable.Such an increased groove depth above 200 nm is also undesirable in viewof the difficulty of transferring the configuration of the groove by aninjection molding technique.

A second aspect of the invention is directed to a method ofsubstantially improving the durability in the file management area whichis more frequently overwritten than other area. Before describing themethod, ATIP signal or ADIP signal which is described by the wobble orTOC will be described more in detail. The ATIP signal described by thewobble is used in controlling the rotational speed of a non-recordedarea and in addressing of data area. (refer to "Compact Disc Dokuhon" byHeitaro Nakajima and Hiroshi Ogawa, published by OHM-sha, third revisededition 1996, "CD Family" cited above and above cited Japanese PatentPublications)

FIG. 6 is a schematic view showing a radial layout of recording area inCD and rewritable CD. A disc region on CD and recordable CD includes aclamping area (a1) which is located along the innermost periphery, whichis followed toward the outer periphery by PCA (Power Control Area) orPMA (Program Memory Area) (a2), a lead-in area (a3), a program area (a4)which corresponds to the data area as termed in the present invention,and lead-out area (a5). A physical location on a track corresponds toabsolute time information of ATIP. A user file is recorded in theprogram area a5 beginning from an origin in time axis which is aninnermost track toward the outer periphery.

As the file is recorded, TOC which describes its address in terms of theabsolute time on the ATIP is entered in the lead-in area a3 whichimmediately precedes it. The beginning position (time) of the lead-inarea is ususally the beginning position (time) of the TOC. Once addressinformation is recorded in terms of EFM modulation signal into TOC, timeinformation of ATIP coincides with the absolute time of subcode-Qchannel which is recorded in terms of EFM modulation data. It is to beunderstood that both ATIP and EFM modulation data describe the absolutetime every 1/75 second.

A unit of such data is referred to as one frame of ATIP, one block ofEFM data or one subcode frame. Since the absolute time and rotationalsynchronization signal of every frame is independent from a datascrambling operation which is performed for the purpose of errorcorrection, they are disposed so that the absolute time proceeds fromthe inner to the outer periphery.

There exists an unrecorded region in the program area until the entireprogram area is filled with user files, and an access to the unrecordedregion can be made with reference to modulated wobbled signal with theabsolute time information of ATIP signal. An access to the recordedregion can be made with reference to the recorded EFM signal with theabsolute time of subcode-Q channel.

Similarly, a control over the rotational speed in the unrecorded regiontakes place by reading a synchronization pattern located at the leadingend of one frame of ATIP signal. In the recorded region, the absolutetime or address information and synchronization information can bedetected from a synchronization pattern of every EFM frame in thesimilar manner as in ROM (Read Only Memory) disc.

Accordingly, once subcode-Q information is recorded in terms of EFMsignal, then absolute time information provided by the subcode-Q channelwill be used, without the need to refer to ATIP signal on wobbledgroove. When retrieving with the ROM drive, only the address informationwhich is recorded in terms of EFM signal and user data are retrieved.

It will be apparent from the preceding description that once the headingfor the beginning position of TOC area (lead-in area) can be detected,then there is no need for ATIP or wobble signal for the TOC. Thus, whatis required in the retrieval of TOC information is only the leadingposition and no reference is made to ATIP signal during the recordingoperation and also during the irirradiation of the recording power.

Since the wobble is provided on an innermost track within the lead-inarea, an synchronization signal from this track may be used to controlthe rotational speed of the disc. When the rotational speed reaches asteady-state there occurs no disturbance which presents a problem tosynchronization between the clock signal and the rotational speed evenif a feedback of synchronization pattern of ATIP signal is unavailablefrom the TOC area. An exact synchronization is detected again at aposition where an access is made to the program area with reference tothe TOC.

FIG. 7 is an illustration of an exemplified CD-RW to which the inventionis applied. Formed on the spiral groove are a preliminary area 22, afile management area (lead-in area) 23 and a data area beginning fromthe inner periphery of the disc. Except for the file management area 23,ATIP is recorded in terms of the groove wobble. The preliminary area 22is used for the purpose of lead-in of a focused light beam adjustment ofthe recording power and/or achieving rotation synchronization.

In the file management area 13, an addressing and a control over therotational speed can be made even without ATIP once the EFM modulateddata is recorded. Specifically, prior to recording of user data, somerecord can be previously made in the file management area in terms ofEFM modulation signal, or alternatively, some means is used to detectthe heading and to make a record only during the initial access to thefile management area as a formatting process on the conventional harddisc drive, and subsequently the recorded signal may be utilized.

To give an example, special information region which records headinglocation information of the file management area as a groove wobble maybe given at a given location in the disc. In this instance, the driveinitially accesses the special information region to detect the headinginformation of the file management area. It is desirable that thespecial information region be located at a position which is initiallyaccessed by the drive, for example, in the preliminary area 12 as shownin FIG. 7. For a disc in which the file management area is located alongthe outer periphery and the drive access begins from the outerperiphery, the special information region may be preferably locatedaround the outer periphery.

As described in JP-A-3(1991)-3168, information for the optimum recordingpower of the disc or information for the drive control may be recordedin superimposition in the ATIP signal of the file management area. Inthis instance, if a degradation due to the repeated overwritingoperations occur, such information can no longer be retrieved correctly.However, such control information can be recorded in the specificinformation region as EFM signal. The recorded information may be onceread by the drive, and then recorded in the file management area in theform of the EFM signal contained in the file management information.

Alternatively, after the manufacture of the disc, EFM modulation signalincluding the absolute time information or the address information maybe recorded in the file management area as an initialization orpost-formatting procedure upon shipment of disc from the factory. Atthis end, a special drive is prepared on the part of a disc manufacture,and, for example, the leading address of the data area may be entered asthe leading address of the unrecorded region. Obviously, otherinformation which are used for the drive control may be recorded in EFMsignal as well. This is preferred because there is no need for a specialfunction in the drive on the user side.

It is also possible to enable an addressing by providing the wobble inthe groove at a point immediately preceding the file management areawithout wobble.

It is desirable that a connection and a synchronization between theabsolute time information in the EFM signal which is recorded in a filemanagement area and the absolute time information of ATIP in thefollowing data area be as smooth as possible, avoiding a discontinuitytherebetween. A technique described in JP-A-3(1991)88124 etc. can beused for such synchronization. Since the file management informationrepresents a table of contents for the user data, it will be apparentthat the less the amount of the user data recorded, the less the filemanagement information needed. However, in accordance with theinvention, it is desirable that an EFM modulation signal be recorded, inthe form of dummy data in the unrecorded region of the file managementarea, in order to provide continuity in the absolute time informationbetween the file management area and the following data area. The dummydata in the file management area may be recorded in any sequence. Forexample, when the file management information is recorded beginning fromthe leading end of the file management area, it is preferred to recordthe dummy data including the synchronization information and addressinformation from the end of file management information to the end offile management area.

Alternatively, for the purpose of providing continuity in the addressinformation, the end of the file management information may be arrangedto coincide with the end of the file management area. In such aninstance, it is preferred that the dummy data be recorded from theleading end of the file management area to the leading end of the filemanagement information. When the initialization or formatting procedurementioned above is effected, only the dummy data may be recorded overthe entire unwobbled file management region.

According to a further aspect of the invention, there is provided anoptical information recording method which uses the disc described aboveand in which the beginning position of recording is displaced each timea part or all of the file management information inclusive of dummyinformation is re-written. It is known that when repeated overwritingoperations take place, it is useful to displace the beginning positionof recording in an incremental manner for the purpose of retarding adegradation in the signal which results from a transfer of the materialin the phase-change disc (see JP-A-2(1990)94113 and -3(1991)-150725). Byapplying this recording method to the file management area of the discof the invention, a degradation in a signal can be reduced. The amountof displacement is limited to a certain degree since it may exceed thepermissible range of the absolute time information. A sufficientimprovement can be obtained with a displacement on the order of 10 to100 μm, for example.

While the invention has been described in relation to CD-RW, it shouldbe understood that the invention is not limited to its use with CD-RW.In addition, the data signal is not limited to the EFM modulationsignal, but may comprise any modulation signal including addressinformation and synchronization information. Providing a deformation inthe signal of the groove geometry is one way of achieving a higherdensity in a recording disc, and is also applicable to a recording dischaving a different format. The invention is also effective in such aninstance. For example, ISO9660 standard is known as the logical formatstandard which also covers CD standards. According to the current CDstandards, only the physical file structure is described for the filemanagement area, and a physical position in unit of data block isdescribed as absolute time information. However, a hierarchicalstructure or so-called directory structure is not described. Accordingto ISO9660, a physical structure is described in the file managementarea, and a directory structure is described in a specified region ofthe data area as a path table. In this instance, it will be evident thatthe path table in this specified region can also be contained in thefile management area as termed in the invention.

JP-A-5(1993)-210849 describes a temporary or transitory recording offile management information in a specified region other than the finalfile management area. The lead-in area a3 shown in FIG. 6 is notrewritten every time, but the file management information storedtemporarily in such a temporary area a2 is rewritten (seeJP-A-5(1993)-210849), it is desirable to treat this region as the filemanagement area to which the invention is applied.

In accordance with the invention, the need to secure and control areplacement region, as by re-recording the file management informationin a replacement region which is not degraded, may be eliminated,thereby greatly facilitating a file control procedure and alsofacilitating a design of the drive and device drivers. Of course, it iswithin the skill of an expert in the art to improve the reliability byusing specification for such replacement regions in combination.

A method of rewriting information in sector by sector as occurs in amagneto-optical disc has not been established for CD-RW. However, ifsuch a method is used also with CD-RW, it is expected that a number oftimes a particular sector is rewritten will amount to one hundredthousand to million times or greater. In such instance, it is expectedthat the present invention achieves a remarkable effect on suppressing adegradation, which results from the repeated overwriting operations, bysimple means and at a reduced cost. When the deformation of the groovein the data area is a periodic meander or wobble, the groove width andthe wobble amplitude should be determined by the following equations:

    0.25≦GW/R.sub.0 ≦0.45 or 0.65≦Gw/R.sub.0(1)

    0.03≦a.sub.w /Gw≦0.08                        (2)

thereby enhancing the overall reliability for durability against therepeated overwriting operations.

A third aspect of the invention relates to a method of incrementallywriting data at random in block unit generally referred to as "packetwriting". Incremental recording in write-once mode is already inpractical use with CD-R, where a data capacity which is recorded at onetime is varying. Since in this application user data is continuouslyrecorded from the inner to the outer periphery, unrecorded region islocated radially outward of a recorded region. There can be no recordedregion which is located radially outside the unrecorded region.Accordingly, it is a simple matter to detect a synchronization and anabsolute time from the EFM signal in the recorded region and to detect asynchronization pattern and an absolute time from the ATIP signal in theunrecorded region.

Recently, in the CD format, the ability is required to record user datain unit of given packet (sector) such as 2^(n) bytes, for example, asoccurring in a hard disc (HD), floppy disc (FD) or a magneto-opticaldisc (MO). A data control to deal with such a sector unit of fixedlength is required in CD-RW in order to take advantage of itsrewritability, since unless overwriting data is physically constrainedwithin a given length, the overwriting may extend to data which are notto be erased.

A demarcation between sectors, sector addresses and synchronizationsignal are previously pre-formatted in terms of pit trains in thesubstrate in MO. drive or post-formatted in terms of recorded signal inHD or FD drives.

FIG. 8 shows an example of sector arrangement for MO. A pit train whichconstitutes a header 25 and a user data area 26 are alternately disposedin the circumferential direction, and one set of the header 25 and thedata area 26 constitutes a sector 27. It is to be noted that the lengthof the header 25 is shown exaggerated in FIG. 8.

In the CD format, a technique of realizing the data control according tothe sector as MO is not yet established. There are proposals, however,which are in public discussion in OSTA (Optical Storage TechnologyAssociation) in the United States. One of the techniques is a proposalreferred to as CD-DASD (Compact Disc Direct Access Storage Disc), inwhich each sector is in unit of 4096 bytes that facilitates thecompatibility with DOS format and which is proposed by Kodak Company inan open presentation of OSTA in February 1996.

Another proposal for a packet writing technique which is to be used withCD-R (see (1) DOS/V Magazine, 1996 June issue, page 214; (2) "CD Family"cited above, Chapter 4; and (3) Nikkei Electronics, Sep. 9, 1996 issue(No. 670), pp. 135-146). A similar packet writing technique is alsodiscussed with respect to the DVD standard of the next generation(Nikkei Byte, 1996 June issue, pp 198-203).

These are requests necessary to construct a logical format to enable anon-sequential recording which does not depend on the operation systemwhether the data format on the disc be CD format, DOS format or else.

As mentioned previously, added information such as synchronizationsignal, address information or the like which are equivalent toinformation described in the sector area is previously recorded on thedisc at a given interval by means of pits, ATIP signal or ADIP signal orthe like. In a typical recordable CD disc, ATIP signal is formed as awobble between the ends of a track without interruption. In order tofacilitate establishing the compatibility with a current CD-R unit, itis desirable to employ address signal contained in ATIP signal, ADIPsignal or EFM signal.

FIG. 9 is a schematic plan view of CD-RW disc according to oneembodiment of the invention. A guide groove 11 used as a data track anda land 12 are alternately disposed in the radially direction, andadditional data area 28 and user data area 29 are alternately disposedin the circumferential direction of each data track 11. A guide groovesection 31 having a wobble which is modulated according to a givensignal is formed in the additional data area 28 whereas a guide groovesection 32 which is not formed with a wobble is formed in the user dataarea 29.

In the CD format, because it is used in a constant linear velocity (CLV)mode, user data having a given byte length corresponds to a given lengthof absolute time. Accordingly, inclusive of added data information, oneunit of user data corresponds to a given length of absolute time.Accordingly, as shown in FIG. 9, a pseudo-sector (or packet) 30comprising a set of preceding additional data area 28'+ user data area29+ following additional data area 28" is laid for each length ofabsolute time T which is described by the ATIP signal in the guidegroove of the disc. Either one of the additional data areas 28' or 28"may be used. It is to be noted that the user data area 19 is in unit of2^(n) bytes, for example.

In the present invention, the set of data is referred to as packet, thephysical structure on the disc on which the packet is recorded isreferred to as pseudo-sector and the additional data area is referred toas a pseudo-header, utilizing the concepts and the terminology used withHD or MO.

When the packet is actually overwritten, attention must be paid to a gapbetween the overwriting packet data and existing data neighboring to theoverwriting data. The drive may give rise to a delicate error in theposition of the beginning or the end of the packet writing in view ofthe rising time of a laser or rotational jitter in drive system when arecording operation actually takes place by the modulation of the laserirradiation. To prevent such an error from destroying information of aneighboring pseudo-sector, it is preferred to provide a gap (buffer)region on the opposite sides of the added data. To give an example, whenpacket writing is performed in unit of 32 Kbyte, it is possible that anerror occurs having a magnitude corresponding to several tens of bytes(see "Nikkei Byte", 1996 June issue, pp.198-203).

In the drive which is performing a CLV operation at a linear velocity V,the length of an pseudo physical sector along the track is equal to aconstant value of VT, apart from the gap, and the additional data area28 is disposed every given interval VT along the track. Since the lengthof each pseudo-sector is constant, it is possible to allocate theabsolute time to the position of leading end of each sector bycalculation, or it is possible to describe it in the lead-in area.

Obviously, this is applicable where the entire disc surface is operatingaccording to CLV as in CD, and a similar access is also possible in ZCLVmode in which zones are demarcated radially of the disc. For a discwhich operates according to CLV or ZCLV, address information may bedescribed by ADIP signal which does not depend on the absolute time. Inthis instance, address information for the leading end of each sector isread from ADIP signal rather than prepits, as occurring in MO discaccording to ISO standard.

The invention relates to a so-called packet writing technique whichutilizes the pseudo-sector, does not directly relate to a detailedtechnique of various proposals, and has for its object an improvement ofthe durability against the repeated overwriting operations in aoverwritable phase-change disc in which a fixed length of data (packet)can be repeatedly recorded on the same pseudo sector.

When applying the invention to a rewritable phase-change disc whichadopts the pseudo-sector structure of the fixed length, the wobble isformed only in the pseudo-header region 28, and not formed in the userdata area 29, as shown in FIG. 9.

Alternatively, at least user data area 29 has a groove formed with thewobble satisfying the following relationship:

    0.25≦GW/R.sub.0≦ 0.45 or 0.65≦GW/R.sub.0(1)

    0.03≦a.sub.w /GW≦0.08.                       (2)

The choice of either configurations depends on a choice in the design toplace greater significance upon either the durability againstoverwriting operations or the improved accuracy in the synchronizationof rotation which is brought forth by the presence of the wobble. If theformer is selected, an excellent durability against overwritingoperations can be obtained when accuracy in the synchronization ofrotation is additionally improved by another method without retardingthe durability against the overwriting operations. If the latter isselected, an excellent accuracy in the synchronization of rotation canbe obtained, although the durability against overwriting operations issomewhat reduced without any practical problem.

Thus, the choice of either embodiments of the present invention isdetermined depending on the philosophy of the design to which one of thedurability against the overwriting operations and the durability againstany defect in the header the preference is to be given. However, in anyevent, a degradation in the durability against overwriting operationswhich is caused by the presence of the wobble found by the presentinventors can be improved.

Specifically, if the latter is selected, added data such as addressinformation can be recorded in the groove or inter-groove in theadditional data region or pseudo-header section as a pre-pit train.Alternatively, an initialization (post-formatting) operation may recordthe address information in the same recording format as the user data inthe additional data area.

If the wobble is not to be formed in the user data area 29, it issufficient that exposure laser irradiation for the groove is leftunmodulated in the user data area 29 during the fabrication process ofthe stamper in order to switch the wobble between the pseudo-headersection 28 and user data area 29. Thus, this can be realized by a simplemodification of an existing mastering signal source.,

As shown in a block diagram of a mastering unit in FIG. 10, theformation of the groove is made possible by opening a gate G1 between amodulation signal generator CM1 which produces a modulation signal usedin forming a wobble, and a laser oscillation drive unit EO which emitsan exposure laser beam for irradiating a prototype glass plate 23. Anarrangement is made to produce absolute time information from amodulation signal MI uninterruptedly, as in the prior art, while asector header switching unit CM2 is made to produce a gate signal M2which causes gate G1 to be opened at the position of the pseudo-header.In this manner, a wobble modulation signal M1 is intermittently suppliedto the EO drive unit.

Even though the laser beam is left unmodulated by closing G1, theabsolute time in the wobble modulation signal M1 is allowed to proceedin CM1 so that the absolute time of each pseudo-sector is an exactfunction of the position when the disc is rotated according to the CLVscheme even though the description of the absolute time is given atintervals.

Only in the user data area where the wobble is not formed, or only inthe user data area where the wobble amplitude is reduced as comparedwith the pseudo-header region, the groove width is reduced by 10 to 50%than in the pseudo-header region so as to satisfy the first relationshipin inequality (1), thereby improving the durability against the repeatedoverwriting operations as the whole disc.

In this manner, a control over the groove width can be accomplishedreadily by controlling the power of exposure laser beam during theexposure of photoresist placed on the prototype glass plate. Thus, at apoint where a switching occurs between the additional data area and theuser data area, the oscillation of the wobble is turned on and off, andthe intensity of the laser beam is switched between two levels.

When recording information to the disc of the present invention byapplying a packet writing technique, address information which isdescribed in terms of synchronization and absolute time is read fromATIP signal in the pseudo-header region, thus initially establishing agiven synchronization of rotation. Subsequently, an address is indexed,followed by a recording operation of the EFM signal over the entirepseudo-sector which begins at the desired absolute time.

During an initial recording of packet, synchronization and absolute timemay be recorded in the wobbled pseudo-header region in terms of the EFMsignal, and subsequently, an access to a desired sector can be made withreference to data represented by the EFM signal without reference toATIP signal.

When retrieving data through an access to the desired pseudo-sector withROM drive which is capable of retrieving data entered by the packetwriting technique, the access to each pseudo-sector can be made byretrieving the recorded EFM signal rather than ATIP signal on thewobble. However, it is undesirable for the purpose of the presentinvention that the EFM data in the wobbled pseudo-header header regionbe rewritten for each packet writing. If the added data is recorded interms of EFM signal in the pseudo-sector, only the user data isoverwritten during a second and a subsequent recording operations.

On the other hand, it is also possible to write only the user data andnot to record in the pseudo-header region in any time including initialwriting. In this instance, a reference to ATIP signal or ADIP signal isrequired to make an access. This requires the addition of aplayback/decode circuit for ATIP signal or ADIP signal to the ROM drive.While this requires additional modification to the drive, it can beeffectively used as an option. This option does not present asignificant problem, since the existing CD family has cleared the demandthat minor additions and/or improvements must be incorporated into thedrive while considering the compatibility with the past technology.

In the event a different ROM standard from the current CD standard isadopted in future in order to increase the density, the presentinvention can be introduced into accommodation on the part of the futureROM drive.

As mentioned previously, it is desirable that a connection and/orsynchronization between the absolute times which are recorded utilizingATIP signal and EFM modulation signal be as smooth as possible, avoidinga discontinuity therebetween. A technique to establish suchsynchronization is disclosed in JP-A-3(1991)-88124. Conversely, during arepeated overwriting operation, it is desirable to displace thebeginning position of recording intentionally at a random within apermissible range in order to retard a degradation in the signal whichmay occur as a result of a material flow which is known to occur with aphase-change disc during repeated overwriting operations (seeJP-A-2(1990)-94113 and -3(1991)-150725). This permissible value is notclearly defined quantitatively at present, but, for the CD format, itmay be estimated from CD-R standard to be on the order of one to two EFMframes (588 channel bit length) or 100-200 μm. A displacement of thebeginning position of the recording within such range is enough toachieve a satisfactory improvement.

During the retrieval of information, an access to each user file is madeby reaching the file management information area first to obtain theaddress information of a given user file, then an access to that addressis actually made, thus reading the user data. The file management areais also referred to as a disc control area. A series of file managementinformation area is usually disposed collectively at a specifiedlocation along the inner or outer periphery of the actual disc.

As a further improvement, the second and third aspects of the inventionmay be combined, thereby substantially completely eliminating theperiodic deformation of the groove in the file management area.

Alternatively, the first and the third aspects of the invention may becombined to provide the wobbled groove in the file management area whichsatisfies the relationships (1) and (2) given above.

The above description has also been given taking an example of CDstandard, for the reason that the definition of terminology is clearlygiven in this instance as mentioned previously. What has beenspecifically discussed above indicates that the present invention is auseful method which can be adopted while maintaining a compatibilitywith the existing CD standard. On the other hand, not only for theexisting CD but also for a recording disc which achieves a higherdensity in the manner of CD, the application of address signal in termsof periodic deformation of the groove geometry can be used incombination with a technique of providing a file management area in aphysically limited region (see Nikkei Byte, 1996 June issue,pp.198-203), and the invention is also effective in this instance. For adisc which is used in CLV or ZCLV mode, it is possible to describe theaddress information in terms of the wobble, and the invention is alsoeffective in this instance.

When a packet writing technique is applied to an overwritablephase-change disc according to the invention, for example, when arecording pulse strategy as illustrated in FIGS. 5A and 5B is used, adegradation of a recording response is little noticeable after 10,000overwriting operations, even though a degradation is noted after 1000repetitions of overwriting operations in the prior art. In someinstances, a degradation in the response was noticed after repetitionson the order of 100,000 times. Accordingly, the need, experienced in theprior art, to secure and control a replacement area in order to allow arewriting of the degraded pseudo-sector can be eliminated or reduced,the pseudo-sector being undergone degradation as a result of repeatedoverwriting operations which happened to occur in this sector in aconcentrated manner. Accordingly, a file control procedure issimplified, also simplifying a design of the drive and the devicedrivers.

A technique of writing/rewriting in sector unit as occurring in amagneto-optical disc is not yet established with CD-RW disc. However, ifsuch technique is established with CD-RW disc, the number of rewritingsis expected to become enormous (for example, up to one hundred thousandto million times or more). It is expected that in such instance, theinvention allows a degradation occurring as a result of repeatedoverwriting operations to be suppressed readily and inexpensively.

As a further application of the invention, a rewritable phase-changedisc will be described below, in which a groove having a configurationmodulated in accordance with rotational synchronization signal is formedand a record is made in both within the groove and the inter-groovespace or land. A method which records information in both the land andthe groove will be abbreviated to hereafter as L&G (Land and Groove)recording.

L&G recording is proposed in JP-B-63(1988)-57859. When such technique isemployed, there is a need to pay a special attention to reducing thecross-talk. Specifically, in the cited Publication, a spacing between atrain of recording marks on a track and a train of recording marks on anadjacent track will be equal to one-half diameter of the focused beam,whereby the train of recording marks on the adjacent track adjacent tothe track to be retrieved will be irradiated by the focused beam. Thisincreases the cross-talk during the retrieval, degrading SIN ratio inthe retrieved signal.

A technique is proposed to reduce the cross-talk by providing a specialoptical system and a cross-talk canceling circuit in an optical discplayback unit, for example (SPIE Vol.1316, Optical Data Storage (1990),p. 35). However, the proposed technique complicates the optical systemand the signal processing system of the playback unit.

There is also another proposal for reducing the cross-talk withoutproviding any special optical system or signal processing circuit. Thetechnique uses providing an equal width for the groove and the land andselecting the groove depth effectively in a range which corresponds tothe wavelength of reproducing light (Jpn. J. Appl. Phys. Vol.32 (1993),pp.5324-5328). According to this proposal, it is shown by calculationand experiments that a reduction in cross-talk can be achieved when thegroove depth is in a range from λ/7n to λ/5n wherein λ and n representthe wavelength of reproducing light and the refractory index ofsubstrate, respectively- This is also described in JP-A-5(1993)-282705.

In order for an amplitude of the recorded signal to be equivalent ineach of the land and the groove, it is required, in addition to LW=GW,that a phase difference α as defined below:

    α=(the phase of a reflected light from a unrecorded region)-(the phase of a reflected light from a recorded region)        (3)

satisfies the following relationship, as described inJP-A-7(1995)-287872:

    (m-0.1) π≦α≦(m+0.1) π            (4)

where m is where m is an integer.

By defining the groove width and the phase difference of the reflectedlight in the manner mentioned above, a desirable recording performanceis obtained for both the land and the groove.

On the other hand, according to the investigation by us, it is foundthat the durability of the land against repeated overwriting operationsdepends on the relative relationship between the land width and the beamdiameter, and when the land width becomes narrower than a specifiedvalue with respect to the beam diameter, a degradation proceeds rapidly.Specifically, if the land width lies in a range form 0.62×(λ/NA) to0.8×(λ/NA), there occurs no failure in erasing previously recorded marksduring repeated overwriting and no substantial degradation of the jitterof the recorded marks, maintaining an equivalent response as occurringduring recording in the groove. However, when the land width is belowthe range described above, a failure in erasing the previous marks isremarkable during the repeated overwriting operation in the land, thejitter of recorded marks is degraded significantly. On the other hand,when the land width exceeds the descried range, there occurssubstantially no problem with respect to the repeated overwritingresponse over the land and an excellent response is achieved. However,it is not preferable to reduce the recording density by increasing theland width without any significant purpose, in view of achieving a highdensity recording.

A further investigation revealed that when the groove width is reducedin order to achieve a higher density with a reduced track pitch whilemaintaining the ratio of the groove width to the land width at almost1:1, repeated overwriting of the track causes the disappearance ofamorphous bit from an adjacent track (which is a groove in either sideof a land if the first mentioned track is represented by the land, andwhich is a land on either side of the groove if the first mentionedtrack is represented by a groove), or causing or recrystallization. Thisphenomenon will be referred to in this text as a cross-erase.

Cross-erase phenomenon depends on a relative relationship between thebeam diameter and the pitch of the recording track. Thus, there exists aminimum track pitch to which the cross-erase can be reduced to a levelwhich is practically negligible, and such minimum pitch depends on onlythe beam diameter.

When the groove pitch (GW+LW) of the L&G recording is selected to begreater than 1.2×(λ/NA) or when the substantial track pitch {(GE+LW)/2}is greater than 0.6×(λ/NA), a degradation in the signal from an adjacenttrack which is caused by the cross-erase can be suppressed, and areduction in the CN ratio after 10,000 times of overwriting operationscan be suppressed below 3 dB, which is a level presenting substantiallyno problem for practical purposes. The theoretical background thereforwill be considered below.

FIGS. 11A, 11B and 11C are a schematic views of a configuration of afocused beam, FIG. 11A showing cross-sectional view of the beam, FIG.11B showing the intensity distribution in a plane, FIG. 11C graphicallyshowing the level of the intensity distribution in FIG. 11B. A focusedbeam 34 which passed through a focusing lens 37 has an intensitydistribution 35 which includes a main peak and sub-peaks. A center spotwhich is represented by the main peak has a diameter which can besubstantially represented as 1.2×(λ/NA), which is referred to as an airydisc 36. The figure of 0.6×(λ/NA) correspond to just half the airy disctheoretically. This allows an assumption of the physical significancethat the cross-erase phenomenon is caused by the fact that the adjacenttrack is raised in temperature by a laser irradiation of reducedintensity which rise in the skirt of the airy disc 36 of the focusedbeam 24 to a first order approximation.

A recording layer of phase-change type which is currently known andprincipally comprises 40 atomic % or more of GeSbTe, AgInSbTe, InSnTe,InSbTe or other IIIb, IVb, Vb or VIb group element either alone or inmixture (namely, as an alloy) has a thermal conductivity which is by twoor three orders of magnitude below that of a magneto-optical recordinglayer. During the time interval on the order of 10 to 100 nanosecondswhich is required for the recording operation, the recording layer issubstantially adiabatic in the lateral direction. Accordingly,cross-erasing phenomenon is little influenced by the thermal conductionof the recording layer. Thus, the minimum track pitch is substantiallydetermined by the beam diameter, and hence by the wavelength of thelight beam and NA alone. However, a modification of multilayer structureof the recording disc and a restriction of the physical properties ofthe recording layer appear to be effective, though slightly, to reducingthe cross-erase after 10,000 times of repeated overwriting operations.

It is noted that those of alloy metal layers mentioned above havingcompositions which permit a reversible change between crystal andamorphous states and which exhibit a reduced degree of cross-erase eraseoften have a melting point Tm below 700° C. and a crystallizationtemperature Tg equal to or above 150° C. Tg below 150° C. makes theamorphous state unstable, thereby causing a cross-erase. Tm equal to orhigher than 700° C. raises the power which must be irradiated during therecording operation, thereby again likely to produce a cross-erasebetween adjacent tracks. Actually, a composition near Ge₁ Sb₂ Te₄ or Ge₂Sb₂ Te₅ has a Tm from 600° C. to 620° C. and Tg from 150° to 170° C. Acomposition Ag₁₁ In₁₁ Te₂₃ Sb₅₅ has a Tm of about 550° C. and Tg ofabout 230° C.

When the thickness of recording layer exceeds 30 nm, the recordingsensitivity is degraded, causing the cross-erase to increases because ofheat transfer to adjacent tracks during the recording operation.

In summary, the groove width GW and the inter-groove space width LW arealso limited by restrictions imposed in connection with a cross-talk,the cross-erase and the durability against the overwriting operations inthe inter-groove space recording. Thus, it is preferable that the groovewidth GW, the width LW of the inter-groove space (i.e., land width) andthe groove depth d satisfy the following relationships:

    0.3 μm≦GW≦0.8 μm                       (5)

    0.3 μm≦LW≦0.8 μm                       (6)

    0.62×(λ/NA)≦LW≦0.8×(λ/NA)(7)

    (GW+LW)/2>0.6×(λ/NA)                          (8)

    λ/7n<d<λ/5n                                  (9)

where λ, n and NA represent the wavelength of a focused light beam, therefractive index of a substrate and the numerical aperture of thefocusing lens, respectively.

There is no problem in applying the first, the second and the thirdaspects of the invention in combination to a phase-change disc having awobbled groove and which undergoes the L&G recording. By combining thisapplication with various conditions mentioned above, the durabilityagainst the repeated overwriting operations can be further improved.

Specifically, when L&G disc is formed with a wobble for detecting therotational synchronization or address, one or all of the first to thethird aspects of the invention may be applied, thus providing a disc ofa high density and a high reliability which has an improved cross-talkand cross-erase resistance and improved durability against repeatedoverwriting operations.

EMBODIMENTS

The invention will now be described more specifically with reference toEmbodiments. However, it should be understood that the invention is notlimited to the Embodiments described below.

Embodiment 1 and Comparative Example 1

Recording disc which was used in an experiment to be described below hada multilayer structure as shown in FIG. 2. Specifically, a quadri-layerstructure including a lower protective layer of ZnS:SiO₂ (200 nm), arecording layer of A_(g5) In₆ Sb₆₀ Te₂₉ alloy (20 nm), an upperprotective layer of ZnS:SiO₂ (20 nm) and a reflective layer of A1₉₈.5Ta₁.5 alloy (200 nm) was formed by a sputtering process. A protectiveovercoat comprising ultra-violet ray cured resin was provided on top ofthe quadri-layer structure.

An overwriting operation was repeated within the groove. Wobble wasformed as an unmodulated signal of 22.05 kHz, and was transferred onto apolycarbonate substrate having a diameter of 120 nm and a thickness of1.2 nm by injection molding technique. The groove had a pitch of 1.6 μm,a width of about 0.5 μm and a depth of about 40 μm. An amorphous markwas formed within the groove. The recording was made by using an opticaldisc drive DDU 1000 manufactured by PULSTEC Company and carrying anoptical head having an NA of 0.55 and emitting a light beam ofwavelength 780 nm and having a beam diameter of 1.35 μm. The recordingwas effected by using a divided pulse technique as illustrated in FIGS.5A and 5B in which a recording power Pw=12 mW, an erasing power Pe=6 mWand a bias power Pb=0.8 mW were used. Measurement was made of adegradation of 3T signal when an EFM random signal was repeatedlyoverwritten by using a double velocity (2.8 m/s) as compared with CD.Signal quality was evaluated in terms of jitter. It is required by theCD standard that the jitter be less than 17.5 nsec at the doublevelocity.

During the initial recording operation, 3T mark jitter was from 9 to 11nsec. By repeating an overwriting operation according to the pulsestrategy, shown in FIG. 5, the number of times (the number ofoverwriting operations) until the 3T mark jitter reaches 17.5 nsec wasdetermined. The CD-RW standard requires a durability in excess of 1,000times, and in the present Embodiment, a disc which demonstrated a numberof times equal to or greater than 1,000 times was regarded asacceptable.

The wobble amplitude was determined according to the technique describedin the orange book. The groove width was determined by the opticaldiffraction method (U-groove approximation). Table 1 indicates thenumber of repeatable times for various values of W/R₀ and a_(w) /W thusobtained. In this table, an area surrounded by a bold line representsresults of this Embodiment, and the remainder relates to the ComparativeExample.

It will be noted that with a groove having no wobble amplitude, littledegradation is noted in the jitter after 5,000 times of overwritingoperations, but the degradation becomes notable with an increase in thewobble amplitude, and at W/R_(O) =0.50, a marked degradation occursafter 1,500 overwriting operations. The progress of the degradation dueto the presence of the wobble is slow when the groove width versus thebeam width is from 0.25 to 0.45, but becomes rapid when this ratio isbelow 0.25 or above 0.45 where the number of repeatable times may bereduced below 1,000 times. When a_(w) /W is equal to or greater than0.08, the degradation during the overwriting operations proceed rapidly.When a_(w) /W is less than 0.03, the wobble exhibits a reduced carrierversus noise ratio. When C/N is less than 25 dB, an accurate retrievalof wobbleing groove signal becomes difficult or a synchronization of therotation of the disc cannot be achieved, thus causing the likelihoodthat address information cannot be read out.

                                      TABLE 1    __________________________________________________________________________    a.sub.w       C/N          GW     0.37 0.42 0.50 0.55 0.67    nm dB GW/R.sub.0                 0.27 0.31 0.37 0.41 0.50    __________________________________________________________________________    32 37 REPETITIONS                 500  1100 2000 1500 800          a.sub.w /GW                 0.086                      0.076                           0.064                                0.058                                     0.048    27 32 REPETITIONS                 1000 1700 2000 1500 1000          a.sub.w /GW                 0.073                      0.064                           0.054                                0.049                                     0.040    20 27 REPETITIONS                 2000 2500 2000 1200 800          a.sub.w /GW                 0.054                      0.048                           0.040                                0.036                                     0.030    13.5       20 REPETITIONS                 2500 2000 3000 1700 1000          a.sub.w /GW                 0.036                      0.032                           0.027                                0.025                                     0.020    0  -- REPETITIONS                 >5000                      5000 3000 3000 1500          a.sub.w /GW                 0    0    0    0    0    __________________________________________________________________________

Embodiment 2 and Comparative Example 2

A substrate was prepared having a track pitch of 1.0 μm, a groove widthof 0.33 μm, a groove depth of 45 μm, and a wobbled groove having anamplitude of 25 μm (a_(w) /W=0.076) with a period corresponding to 22.05kHz was prepared. A quadri-layer structure including a lower protectivelayer of ZnS:SiO₂ (150 nm), a recording layer of Ge₂₃ Sb₂₅ Te₅₂ alloy(20 nm), an upper protective layer of ZnS:SiO₂ (20 nm) and a reflectivelayer of A1₉₈.5 Ta₁.5 alloy (100 nm) was produced by a sputteringprocess. A protective coat of ultra-violet ray cured resin was providedon the quadri-layer structure.

An overwriting operation was repeated within the groove at a linearvelocity of 2.8 m/s in the similar manner as in Embodiment 1.

In the Embodiment 2, the beam had a wavelength of 680 nm, NA=0.6 and R₀=1.05 μm, with a recording power Pw=11 mW, an erasing power Pe=4 mW anda bias power Pb=0.8 mW according to the divided pulse technique.

On the other hand, in the Comparative Example 2, the beam had awavelength of 780 nm, NA=0.55 and R₀ =1.35 μm, with a recording powerPw=13 mW, an erasing power Pe=6 mW and a bias power Pb=0.8 mW accordingto the same pulse strategy. In either optical system, the wobble had aC/N ratio which was equal to or greater than 25 dB.

In the Embodiment 2, W/R₀ =0.31 while in the Comparative Example 2, W/R₀=0.24. In the Embodiment 2, the overwriting operations could be repeated5,000 times and more. On the other hand, in the Comparative Example 2,repeatable times was on the order of 700, and after several hundredtimes, a significant reduction was recognized in the C/N ratio of thewobble.

Comparative Example 3

A recording disc having the same layer structure as described above inconnection with the Embodiment 1 was formed except for the groove widthof 0.53 μm, the groove depth of 20 nm and the wobble amplitude of 27 nm.In the evaluation which used a similar optical system as described inEmbodiment 1, the number of repeatable times was on the order of 500. Itis considered that this is attributable to a shallow groove depth.

Comparative Example 4

A substrate was prepared in a similar manner as described above inconnection with the Embodiment 1 except for the groove width of 0.53 μm,the groove depth of 30 nm and the wobble amplitude of 27 nm. The similarlayer structure was prepared as above except for the lower protectivelayer having a thickness of 60 nm.

In the evaluation which employed a similar optical system, it was foundthat the number of repeatable times was on the order of 500. It isconsidered that this is attributable to a thin thickness of the lowerprotective layer.

Comparative Example 5

A substrate was prepared in a similar manner as described above inconnection with the Embodiment 1 except for the groove width, groovedepth and wobble amplitude of 0.53 μm, 35 nm and 27 nm, respectively.The similar layer structure was used as above except that for the upperprotective layer having a thickness of 65 nm.

In the evaluation which employed the similar optical system, it wasfound that the number of repeatable times was on the order of 800. It isconsidered that this is attributable to a too thick upper protectivelayer.

Embodiment 3

Formed in a polycarbonate substrate which was injection molded to adiameter of 120 nm and a thickness of 1.2 mm was a spiral groove havinga pitch of 1.6 μm, a width of about 0.5 μm and a depth of about 40 nm. Awobble with a signal of 22.05 kHz was formed in the groove. The wobblewas formed in four types having amplitudes of 27 nm, 20 nm, 13.5 nm and0 nm (no-wobble).

Sequentially laminated on the substrate were a lower protective layer of(ZnS)₈₀ (SiO₂)₂₀ (mol %) to a thickness of 100 nm, a recording layer ofAg₅ In₆ Sb₆₁ Te₂₈ to a thickness of 20 nm, an upper protective layer of(ZnS)₈₀ (Sio)₂₀ to a thickness of 20 nm and finally a reflective layerof A1₉₇.5 Ta₂.5 to a thickness of 100 nm. Ultra-violet cured resin(SD318 manufactured by Dainippon Ink.) was coated on the resultingstructure to a thickness of several pm, thereby preparing a phase-changedisc. A recording was made within the groove, forming amorphous marks inthe crystallized region.

The disc was repeatedly overwritten in an accordance with EFM randomsignal using a velocity (2.8 m/s) which was double the velocity usedwith CD.

The recording operation was performed by using an optical disc driveDDU1000 manufactured by PULSTEC Company and carrying an optical headhaving an NA of 0.55 and emitting a light beam of wavelength 780 nm andhaving a beam diameter R₀ of 1.35 μm.

An overwriting operation is effected by using a divided pulse techniqueas indicated by a laser irradiation pattern shown in FIGS. 5A and B witha recording power Pw of 11 mW, an erasing power Pe of 6 mW and a biaspower Pb of 0.8 mW. The signal quality was evaluated in terms of 3Tjitter which was most stringent. It is required by the CD standard thatthe jitter be on the other of 17.5 nsec or less at the double velocity.

FIG. 12 graphically shows results of measurement of 3T jitter for wobbleamplitudes of 27 nm, 20 nm, 13.5 nm and 0 nm or no-wobble. It will beapparent from this Figure that for a groove which has no wobble, thereis little degradation of jitter after 10,000 times of overwritingoperations, but the degradation increases markedly with an increase inthe wobble amplitude, and becomes remarkable at a point whichcorresponds to the order of 1,000 times. It is also to be noted that thedegree of degradation caused by the repeated overwriting operations alsodepends on the cross-sectional geometry of the groove.

FIG. 13 graphically illustrates a degradation of the recording responsecaused by wobbled groove which is arrayed at pitch of 1.6 μm. The groovedepth remains constant, but the groove width alone is changed between0.40 μm, 0.56 μm, 0.68 nm and 0.80 μm. In this instance, a groove widthW as measured at half the groove depth H/2 is taken as the effectivegroove width, as indicated in FIG. 15.

It will be readily understood that within an extent of the times ofoverwriting operations illustrated in FIG. 13, in the presence of thewobble, a degradation during repeated overwriting will be retarded forgreater groove widths. On the other hand, in the absence of the wobble,a degradation during repeated overwriting will be retarded for smallergroove widths.

Embodiment 4

A polycarbonate substrate having a spiral groove was prepared byinjection molding. The refractive index at a wavelength of 680 nm was1.56. Both the groove width and the land width were 0.75 μm while thegroove depth was about 70 nm. A lower dielectric protective layer, arecording layer, an upper dielectric protective layer and a reflectivelayer were sequentially formed on the substrate by sputtering.

Each of the lower and upper dielectric protective layer contained(ZnS)₈₀ (SiO₂)₂₀ , and had a thickness of 100 nm and 20 nm,respectively. A material for the recording layer contained as maincomponents thereof Ge, Sb and Te, which were subjected to a reversiblephase-change between amorphous and crystallized states in response to alaser irirradiation, and the composition of Ge:Sb:Te was in the atomicratio of 2:2:5. The recording layer had a thickness of 25 nm. Thereflective layer contained A1₉₇.5 Ta₂.5 deposited to a thickness of 100nm. A ultra-violet ray cured resin was coated on the reflective layer asa protective overcoat.

Since the recording layer generally assumes an amorphous state in theas-deposited state, the entire surface of the recording layer wasannealed by the laser irradiation to cause a phase-change into thecrystallized state, which represents the initial or unrecordedcondition.

A recording operation was effected by irradiating with a focused beamfrom a high power laser to the track, thereby changing the recordinglayer into the amorphous state. A change in the amount of reflectedlight from resulting amorphous recorded marks was used to detect therecorded marks.

The disc is then rotated at the linear velocity of 10 m/s, and asemiconductor laser diode beam having a wavelength of 680 nm was focusedonto the recording layer through the objective lens having a numericalaperture of 0.60. The beam diameter R_(O) is equal to 1.05 nm. Arecording and a retrieval of a signal is made while performing atracking control in the push-pull arrangement.

An arbitrary groove was first selected, and a signal having a frequencyof 7.47 MHz was recorded therein. The recording power was changedbetween 10 mW and 12 mW in increment of 1 mW while both erasing powerand bias power were maintained at 6 mW, thus performing one-beamoverwriting operation. Subsequently, a retrieval was made, and adesirable C/N ratio of 54-55 dB was measured with a spectrum analyzerhaving a resolution bandwidth of 30 kHz. Then an arbitrary land wasselected to make a similar record and a similar measurement of C/Nratio, which was found to be 54-55 dB which is substantially same as inthe groove.

It was found that the noise level during recording on the land and thenoise level during recording in the groove were comparable. Accordingly,a comparison of C/N ratio was synonymous with a comparison of a recordedcarrier level.

A phase difference α between light reflections between crystallizedstate and the amorphous state of the recording layer is calculated to be0.01 π.

Embodiment 5

The same disc as used in the Embodiment 4 was rotated at a linearvelocity of 15 m/s, and an arbitrary groove is chosen to record a signalhaving a frequency of 11 MHz by using the same recording apparatus asused in the Embodiment 4. A one-beam overwriting operation was performedwith a recording power of 12 mW, and an erasing power and a bias powerboth of which were equal to 7 mW. C/N ratio was equal to 52 dB.

Subsequent to the recording operation, the recorded track was irradiatedwith DC laser irradiation having power of 7 mW, whereupon the carrierlevel was reduced by 25 dB, indicating a desirable erasability asrepresented by an erasure ratio of 25 dB. Subsequently, an arbitraryland was selected to make a similar record and a similar measurement ofC/N ratio. It was found that C/N ratio was equal to 52 dB which issubstantially same as in the groove. An erasure ratio in this instancewas also equal to 24 dB, which was comparable to that obtained for thegroove.

Embodiment 6

A disc was prepared in a similar manner as described above in connectionwith the Embodiment 4 except that the recording layer had a compositionof Ge₂₂ Sb₂₅ Te₅₃. A stoichiomertic composition of Ge:Sb:Te=2:2:5 ispreferred for use with a recording retrieval at or greater than a linearvelocity of 10 m/s. For a linear velocity below 10 m/s, it is effectiveto increase the amount of Sb slightly in order to prevent a distortionin the configuration of recorded marks due to the re-crystallization.

The disc was rotated at a linear velocity of 3 m/s, and an arbitrarygroove was selected to record a signal having a frequency of 2.24 MHzusing the same apparatus as used in the Embodiment 4. A one-beamoverwriting operation was performed while changing the recording powerbetween 7 mW and 11 mW in increment of 0.5 mW and using an erasing powerand a bias power both of which were equal to 4.5 mW. During a subsequentretrieval, a determination with the resolution bandwidth of 10 kHzrevealed a desirable C/N ratio of 57-59 dB.

Subsequently, an arbitrary land is selected to make a similar record anda similar determination of C/N ratio, which was found to be 57-59 dB, insubstantially the same manner as for the groove.

FIG. 16 graphically shows the relationship between C/N ratio and therecording power obtained with this Embodiment.

A calculation of a phase difference between light reflections from thecrystallized and the amorphous state of the recording layer indicatedthat the reflected light from the amorphous state advances in phase by0.01 π.

Comparative Example 6

A disc is prepared in quite the same manner as mentioned in connectionwith Embodiment 6 except that the recording layer has a thickness of 20nm. The disc was rotated at a linear velocity of 3 m/s, and an arbitrarygroove was selected to record therein a signal having a frequency of2.24 MHz using the same apparatus as used in Embodiment 4. A one-beamoverwriting operation is performed while changing the recording powerbetween 5 and 10 mW in increment of 1 mW and while maintaining theerasing power and the bias power constant at 4.5 mW. A desirable C/Nratio of 56 dB was obtained.

Subsequently, an arbitrary land was selected to make a similar recordand a similar measurement of C/N ratio, which was found to be 53 dB. Inthis manner, the signal quality was no longer equivalent between theland and the groove, producing a difference in the C/N ratio which wasas large as 3 dB due to a large phase difference α.

A calculation of a phase difference between light reflections from thecrystallized and the amorphous states of the recording layer revealedthat the reflective light from the amorphous state is advanced by 0.20π. An absorption ratio A_(c) /A_(a) of the recording layer is calculatedto be 0.85.

Comparative Example 7

A disc is prepared in quite the same manner as in Embodiment 6 exceptfor the lower dielectric protective layer having a thickness of 180 nm,the recording layer a thickness of 20 nm and the upper dielectricprotective layer a thickness of 80 nm.

The disc is rotated at a linear velocity of 3 m/s, and an arbitrary landwas selected to record a signal having a frequency of 2.24 MHz using thesame apparatus as used in Embodiment 4. A one-beam overwriting operationis performed while changing the recording power between 8 mW and 9 mW inincrement of 0.5 mW and while maintaining the erasing power and the biaspower constant at 4.5 mW. A C/N ratio of 50-51 dB was obtained.

Subsequently, an arbitrary groove was selected to make a similar recordand a similar measurement of C/N ratio, which was found to be as low as39-40 dB. In this manner, the signal quality was drastically degradedfor one of the land and the groove, producing a difference in the C/Nratio therebetween which was as much as 11 dB.

FIG. 17 graphically shows a relationship between the C/N ratio and arecording power of the disc control.

A calculation of a phase difference a between light reflections from thecrystallized and the amorphous states of the recording layer revealedthat the reflected light from the amorphous state was lagging by 0.16 π.This large phase difference α must be responsible for the imbalance ofC/N ratio between the groove and land recordings.

Comparative Example 8

A disc was prepared in substantially the same manner as described abovein connection with Embodiment 6 except for the lower dielectricprotective layer having a thickness of 220 nm, the recording layer athickness of 20 nm and the upper dielectric protective layer a thicknessof 80 nm.

The disc was rotated at a linear velocity of 3 m/s, and an arbitraryland was selected to record a signal having a frequency of 2.24 MHz withthe same apparatus as used in Embodiment 4. A one-beam overwritingoperation was performed while changing the recording power between 5 mWand 9 mW in increment of 0.5 mW and while maintaining the erasing powerand the bias power constant at 4.5 mW. A desirable C/N ratio of 51-52 dBwas obtained.

Subsequently, an arbitrary groove was selected to make a similar recordand a similar measurement of C/N ratio, which was found as low as 44-45dB. In this manner, the signal quality was drastically degraded for oneof the land and the groove, producing a difference in the C/N ratiowhich was as much as 7 dB. A calculation of a phase difference betweenlight reflections from the crystallized and the amorphous states of therecording layer revealed that the reflective light from the amorphousstate was lagging by 0.25 π.

Embodiment 7

A plurality of spiral grooves were provided on a substrate, changing thegroove pitch between 1.1 μm and 1.6 μm in increment of 0.5 μm. In eachinstance, the groove width and the land width were equal to each other.A substantial recording track pitch in terms of L&G recording was from0.55 to 0.8 μm. The groove depth was 70 nm.

Sputtered on this substrate were a lower dielectric protective layer, arecording layer, an upper dielectric protective layer and a reflectivelayer. The layer structure remained same as in Embodiment 4 except thatthe recording layer had a composition of Ge₂₂ Sb₂₃.5 Te₅₄.5. The discwas repeatedly overwritten for evaluation of the cross-erase.

A calculation of a phase difference between light reflections from thecrystallized and amorphous states of the recording layer revealed thatthe reflected light from the amorphous state advanced by 0.01 π.

Initially, the recording was made either in the groove or the land, andthen a pair of adjacent lands or grooves were repeatedly overwritten inorder to measure a reduction in C/N ratio of the signal which wasrecorded in the initial groove or land.

The disc was rotated at a linear velocity of 10 m/s, and semiconductorlaser diode beam having a wavelength of 680 nm was focused on arecording layer through an objective lens having a numerical aperture of0.60, thus recording and retrieving a signal while performing a trackingcontrol in the push-pull method. The beam diameter R₀ was equal to 1.05μm.

Initially an arbitrary groove was selected, and a signal having afrequency of 2.24 MHz was recorded with a duty cycle of 25%. A one-beamoverwriting operation was performed using a recording power of 8/9 mWand an erasing power and a bias power both of which were equal to 4.5mW.

As a consequence, it was found that when a groove pitch was equal to orgreater than 1.45 μm (or corresponding to a track pitch of 0.725 μm), areduction in the C/N ratio between the adjacent grooves or lands after10,000 times of overwriting operations could be suppressed less than 3dB which presented substantially no problem for practical purposes.

A result of analysis conducted by the present inventors based on thenumerical solution of the thermal diffusion equation indicated that themultilayer structure employed in this Embodiment represented one of mostsignificant lateral thermal diffusions, and therefore, the describeddetermination investigated the cross-erase under the most stringentcondition. Accordingly, any other multilayer structure which had theminimum track pitch mentioned above or a greater track pitch presentedsubstantially no problem.

Embodiment 8

A disc as used in Embodiment 8 was used to overwrite a land repeatedlyin order to determine a mark length jitter of the signal on the land.The recording and retrieval conditions are similar to Embodiment 8except that the focusing lens had a numerical aperture of 0.55. The beamdiameter R₀ was equal to 1.15 μm.

For only the groove pitch of 1.55 μm and 1.6 μm (corresponding to trackpitches of 0.775 μm and 1.8 μm, respectively), an increase in a jitterafter 10³ times of overwriting operations was suppressed to the order of20%.

On the other hand, for an groove pitch of 1.4 μm (corresponding to antrack pitch of 0.7 μm) the jitter increases substantially, and wasdoubled or more after 10³ times of overwriting operations.

What is claimed is:
 1. An optical disc comprising a substrate havingthereon a spiral groove or concentric grooves for guiding a focusedlight beam and a land between each adjacent turns of said groove orbetween adjacent said grooves, at least a part of said groove or grooveshaving a wobble in accordance with a modulation signal and having adepth between 25 nm and 200 nm, and at least three layers including alower protective layer having a thickness between 70 nm and 200 nm, arewritable recording layer of phase-change type, and an upper protectivelayer having a thickness between 10 nm and 60 nm, wherein said wobbleprovides a wobble signal having a ratio of C/N which is not lower than25 dB, and wherein parameters of said optical disc and the focused lightbeam satisfy the following relationship:

    0.25≦GW/R.sub.0 ≦0.45,

or

    0.65≦GW/R.sub.0,

and

    0.03≦a.sub.w /GW≦0.08,

where a_(w), R₀ and GW represent a wobble amplitude, a beam diameter ofthe focused light beam measured across said groove and a groove width,respectively.
 2. An optical disc according to claim 1 wherein saidoptical disc has a data recording area in said groove or grooves andsaid land, the focused light beam has a wavelength not longer 700 nm,and the parameters of said optical disc and the focused light beamfurther satisfy the following relationship:

    (m-0.1)π≦α≦(m+0.1)π

    0.3 μm≦GW≦0.8 μm;

    0.3 μm≦LW≦0.8 μm;

    0.62×(λ/NA)≦LW≦0.8×(λ/NA);

    (GW+LW)/2>0.6×(λ/NA);

    λ/7n<d<λ/5n;

    where α=(phase of reflected light from unrecorded region)-(phase of reflected light from a recorded region),

and where λ, LW, n, NA, m and α represent a wavelength of the focusedlight beam, land width, a refractive index of said substrate, anumerical aperture of a focusing lens used for the focused light beam,an integer and the groove depth, respectively.
 3. An optical discaccording to claim 1 wherein said groove defines a data track along saidgroove, said data track including a user data area and an additionaldata area alternately disposed in a circumferential direction of saiddisc, said relationship holds at least wobbles in said user data area.4. An optical disc according to claim 3 wherein said additional dataarea has an additional data implemented as a pre-pit train.